1. Field of the Invention
The present invention relates to a semiconductor device, and in particular to a semiconductor device for use in a Static Random Access Memory (SRAM) in which data (potential) is retained and accumulated by cross-wiring of inverter circuits or in flip-flop circuits in logic circuits, etc.
2. Description of the Related Art
As disclosed in Japanese Patent Laid-Open No. JP07-263577A, an SRAM with a flip-flop circuit comprises two inverters each including a p-type MOS (Metal Oxide Semiconductor) transistor and an n-type MOS transistor. In the inverter circuits, drains of the p-type MOS transistor and the n-type MOS transistor are connected to each other, and gates of the p-type MOS transistor and the n-type MOS transistor are connected to each other. Moreover, sources of these transistors are connected to the power-source potential and the ground potential, respectively. The cross wiring of such the two inverter circuits may form one SRAM cell. In the SRAM of this configuration, let “1” be the case where one of the nodes is in high-level and let “0” be the case where the node is in low-level, which enables storing of binary information (potentials).
On the other hand, in such a semiconductor device including an SRAM, some situations may arise where an α-ray would be incident in a semiconductor device, which is radiated from a radioactive element such as U or Th contained in the package or solder. In this case, a phenomenon is known that many pairs of electrons and holes are produced, the effect of which could inverse the stored information (potentials) and impose some errors. These types of errors are referred to as “soft-errors” as they can be corrected by rewriting of signals and be of transient malfunction.
Specifically, as for metal as a conductor, electrons and holes that are generated by α-rays should not move within the metal of conductors since the inner potentials remain constant. In addition, as for insulators, electrons and holes that are generated by α-rays could not move within the insulators.
However, for semiconductors with p-n junctions, when a reverse bias voltage is applied to the p-n junction, electrons and holes that are generated at the time of transmission of α-rays would move to the positive electrodes and to the negative electrodes respectively, each of which leading to the noise current. Therefore, the semiconductor region is susceptible to the soft-errors, especially in the region adjacent the region of the p-n junction.
Meanwhile, the recent improvement of technology for removing radioactive elements contained in the package material or solders have reduced the problems of soft-errors than before that are caused by α-rays. On the other hand, neutron rays, which are contained in cosmic rays, have attracted public attention as the cause of occurrence of soft-errors. This means that the neutron rays, which are contained in the cosmic rays falling onto the ground, are incident on the semiconductor material such as Si, and collide with a Si atom or the like that resides within the material, destroying the atomic nucleus to produce high-energy secondary particles. These high-energy secondary particles then pass through the semiconductor, thereby generating pairs of electrons and holes as in the α-rays, which could cause soft-errors.
The difference between the soft-error caused by neutron rays and the other caused by α-rays is that the neutron rays have about ten times higher density of pairs of electrons and holes per unit-length than the α-rays. The higher the altitude from the ground, the more the neutron rays would exist. Consequently, in the semiconductor devices such as SRAM mounted on airplanes, the soft-errors, especially those caused by the neutron rays, would become a problem.
Actually, for the α-rays, it is possible to eliminate the elements of the sources and to deal with the externally incident sources by means of shields, etc., and the countermeasures are apparent since their sources have been already identified. However, for the neutron rays, although they are contained in the cosmic rays and are of externally incident nature, the neutron rays offer such an extremely high transmittance as compared to the α-rays that is nearly impossible to shield against. As such, there is no apparent resolution.
Moreover, as the degree of semiconductor device integration increases with the progress of refinement, their soft-error tolerance tends to be reduced for the more refined semiconductor devices. Therefore, it can be thought that this problem would gain increased attention in the future. Patent documents of Japanese Patent Laid-Open No. JP07-263577A and JP2005-12328A disclose inventions related to the semiconductor devices with an increased soft-error tolerance.